1. Field of the Invention
The present invention relates to an improved method for color Doppler flow mapping of cardiac blood flow.
2. Description of the Related Art
Color Doppler flow mapping was developed in the early 1980's as a two dimensional extension of the basic pulsed wave Doppler ultrasound concept. In clinical application, Doppler ultrasound instruments allow noninvasive measurement of blood cell velocities, in which ultrasound waves (&gt;20,000 Hz) are emitted from a transducer and pass into the cardiac chambers where they strike moving blood cells. Part of the ultrasonic wave is then reflected back to the transducer. The frequency of emitted sound is known and the frequency of the returning signal is measured. The Doppler principle then allows conversion of this frequency shift into the blood cell velocity by the equation EQU V=(cf.sub.d)/(2f.sub.0 COS.theta.) (1)
where
v=Velocity of the target PA1 c=Speed of sound in the medium (approx. 1540 m/s in tissue/blood) PA1 f.sub.d =Frequency shift, or Doppler shift, equal to frequency difference between emitted and reflected sound PA1 f.sub.o =Emitted frequency PA1 .theta.=The Angle between ultrasound beam path and blood cell velocity vector PA1 c=speed of sound in the medium PA1 .DELTA.t=time between emission and reception of sound
By pulsing a burst of ultrasound, waiting a time t and then monitoring the returning signal, the depth from which the signal must have returned can be calculated as EQU D=c (.DELTA.t)/2 (2)
where
Conventional pulsed wave Doppler ultrasound refers to application of this principle to measurement of velocity in a single sample volume at a desired distance away from the transducer. Color Doppler flow mapping allows extension of the concept to provide a matrix of velocities as follows. A burst of ultrasound is emitted and then monitored incrementally as it returns, basically dividing the .DELTA.t in equation (2) into intervals and providing velocities at a series of locations along a single line. This "string" of sample velocities is then sequentially refocused through a range of angles from a multi-crystal transducer, ultimately resulting in a pie shaped matrix of velocities. This pie shaped matrix can be updated 8 to 30 times in a second with conventional instruments. As it is not possible to examine individual spectra from the thousands of measurement locations, velocities are averaged within a sample volume and color coded using a predetermined color bar. These color coded velocities are then finally superimposed on an echocardiographic image, which may be thought of as an ultrasound "radar" providing position data on solid cardiac structures.
A significant contribution of this technology is in the detection of abnormal lesions in heart disease. For example, the basic function of a heart valve is to allow forward flow with little obstruction when the valve is open and to prevent backflow when closed. Valves often develop lesions or close improperly, allowing undesired blood to pass in a reverse direction across the valve. Fluid mechanically this condition is reflected by a turbulent jet which in the setting of mitral regurgitation, for example, would emerge from a leaking mitral valve and enter the left atrium. On a color Doppler flow map the condition is reflected by a multicolored jet-type image in the left atrium. Two factors influence the image produced. First, the entire receiving chamber is filled with nonzero velocities and in light of the above explanation, the entire chamber should be filled with color, but it is not. Instead, a tear-drop shaped jet appears. Second, this shape appears because of a high pass filter present in the data processing circuitry of the instrument.
This high pass filter, referred to as a wall filter, is set so as to eliminate low frequency, high amplitude signals returning from cardiac structures. In other words, the moving structures have nonzero velocity just as the blood cells do and therefore produce an appreciable Doppler shift. However, due to the large size of the structures (on the order of centimeters) compared to blood cells (on the order of microns), they send signals back to the transducer which have a very high amplitude and these signals can saturate image processing circuitry. These filters were employed in conventional pulsed wave Doppler equipment in order to eliminate the noise produced by the moving cardiac structures. With the advent of color flow mapping the filters naturally carried over as the threshold which defines a color jet boundary. Detected frequency shifts above the filter receive color encoding, and those below it do not.
The ability to visualize regurgitant jets with color Doppler flow mapping led to great enthusiasm in the mid-eighties. In numerous studies, jet size by color Doppler was related to the severity of regurgitation and compared to the gold standard of catheterization. Planimetered jet areas became commonly used in many centers at least as a marker of the severity of regurgitation. However, studies over the last four to five years have revealed that such an approach is dangerous due to the fact that jet size determined by color Doppler is tremendously variable as a function of factors which are independent of regurgitant flow. These factors are both (a) technical, such as gain settings, frame sampling rate, and (b) physical in nature, such as driving pressure, interrupting flows, heart rate interaction with frame rate. Approaches based on engineering principles of conservation of momentum and mass have been advanced, but have been difficult to apply in the clinical setting Furthermore, the clinician is more inclined to embrace a technique requiring simplified measurements of jet area as opposed to one which requires more cumbersome engineering calculations.
Current filter settings are chosen to eliminate noise resulting from cardiac structures. This unfortunately results in the jet boundaries being defined in relatively unstable portions of the jet.
There remains, therefore, a substantial need for a method of color Doppler mapping of cardiac blood flow and other vascular blood flow, which eliminates the problems set forth hereinbefore.